The present invention is generally directed to a system for receiving a spread spectrum signal and specifically to a system for receiving and demodulating a spread spectrum signal.
Spread spectrum techniques are finding larger roles in a variety of applications. In cellular telephony, spread spectrum based systems offer the potential for increased efficiency in the use of bandwidth. The resistance of spread spectrum methods to jamming make them ideally suited for radar and Global Positioning System (GPS) applications. For radar applications, spread spectrum signals have a lower probability of being intercepted due to the noise-like appearance of spread spectrum waveforms. In addition, it may be used to increase the pulse repetition frequency without sacrificing unambiguous range.
In spread spectrum radars, GPS, and cellular telephony applications (e.g., Code Division Multiple Access (CDMA)), each transmitted signal or pulse is assigned a time varying pseudo-random code that is used to spread each bit in the digital data stream (i.e., an interference code), such as the long code and PN code in CDMA applications. In CDMA applications, this spreading causes the signal to occupy the entire spectral band allocated to the Multiple Access System (MAS). The different users in such a system are distinguished by unique interference codes assigned to each. Accordingly, all users simultaneously use all of the bandwidth all of the time and thus there is efficient utilization of bandwidth resources. In addition, since signals are wide-band, the multipath delays can be estimated and compensated for. Finally, by carefully constructing interference codes, base-stations can operate with limited interference from adjacent base stations and therefore operate with higher reuse factors (i.e., more of the available channels can be used).
In spread spectrum systems, all other spread spectrum signals contribute to background noise, or interference, relative to a selected spread spectrum signal. Because each user (or radar pulse or GPS satellite signal) uses a noise-like interference code to spread the bits in a signal, all the users contribute to the background noise. In CDMA systems in particular, user generated background noise, while having a minimal effect on the forward link (base-to-mobile) (due to the synchronized use of orthogonal Walsh Codes), has a significant effect on the reverse link (mobile to base) (where the Walsh Codes are commonly not synchronized and therefore nonorthogonal). The number of users a base-station can support is directly related to the gain of the antenna and inversely related to the interference. Gain is realized through the amplification of the signal from users that are in the main beam of the antenna, thereby increasing the detection probability in the demodulator. Interference decreases the probability of detection for a signal from a given user. Although xe2x80x9ccodexe2x80x9d filters are used to isolate selected users, filter leakage results in the leakage of signals of other users into the signal of the selected user, thereby producing interference. This leakage problem is particularly significant when the selected user is far away (and thus the user""s signal is weak) and the interfering user is nearby (and thus the interfering user""s signal is strong). This problem is known as the near-far problem.
There are numerous techniques for improving the signal-to-noise ratio of spectrum signals where the noise in the signal is primarily a result of interference caused by other spread spectrum signals. These techniques primarily attempt to reduce or eliminate the interference by different mechanisms.
In one technique, the interfering signals are reduced by switching frequency intervals assigned to users. This technique is useless for the intentional jamming scenario in which jammers track the transmitter frequencies. Frequency switching is not an option for the CDMA standard for cellular telephones. In that technology, all users use all of the frequencies at all times. As a result there are no vacant frequency bands to switch to. In another technique, the interfering signals are selectively nulled by beam steering. Classical beam steering, however, does not provide, without additional improvements, the required angular resolution for densely populated communications environments.
The above techniques are further hampered due to the fact that signals rarely travel a straight line from the transmitter to the receiver. In fact, signals typically bounce off of buildings, trees, cars, etc., and arrive at the receiver from multiple directions. This situation is referred to as the multipath effect from the multiple paths that the various reflections that a signal takes to arrive at the receiver.
An objective of the present invention is to provide a system architecture for increasing the signal-to-noise ratio (SNR) of a spread spectrum signal. Another objective is to provide a system architecture for removing the interference from a spread spectrum signal, particularly the interference attributable to spread spectrum signals generated by other sources. Yet another objective is to provide a system architecture for removing the interference from a spread spectrum signal that does not employ beam steering. Specific related objectives include providing a demodulating/decoding system for efficiently demodulating/decoding spread spectrum signals generated by far away sources in the presence of spread spectrum signals generated by near sources and/or effectively accounting for the various multipaths of a spread spectrum signal.
These and other objectives are addressed by the spread spectrum system architecture of the present invention. In a first configuration of the present invention, the system includes: (i) an antenna adapted to receive a signal that is decomposable into a first signal segment and a second signal segment, the first signal segment of the signal being attributable to a first source and the second signal segment of the signal being attributable to a source other than the first source; and (ii) an oblique projecting device, in communication with the antenna, for determining the first signal segment. The signal can be any structured signal, such as a spread spectrum signal. A xe2x80x9cstructured signalxe2x80x9d is a signal that has known values or is created as a combination of signals of known values.
The oblique projecting device determines the first signal segment by obliquely projecting a signal space spanned by the signal onto a first space spanned by the first signal segment. As used herein, the xe2x80x9cspacexe2x80x9d spanned by a set xe2x80x9cAxe2x80x9d of signals is the set of all signals that can be created by linear combinations of the signals in the set xe2x80x9cAxe2x80x9d. For example, in spread spectrum applications, the space spanned by the signals in set xe2x80x9cAxe2x80x9d are defined by the interference codes of the one or more selected signals in the set. Thus the space spanned by interfering signals is defined by all linear combinations of the interfering signals. The signal space can be obliquely projected onto the axis along a second space spanned by the second signal segment. The estimated parameters of the first signal segment are related to the actual parameters of the first signal segment and are substantially free of contributions by the second signal segment. Through the use of oblique projection, there is little, if any, leakage of the second signal segment into computed parameters representative of the first signal segment.
For spread spectrum applications where noise characteristics are quantifiable, oblique projection is preferably performed utilizing the following algorithm:
(Ixe2x88x92S(STS)xe2x88x921ST)H(HT(Ixe2x88x92S(STS)xe2x88x921ST)H)xe2x88x921HT(Ixe2x88x92S(STS)xe2x88x921ST)
and hypothetical correlation functions generated using the following equation:
(yT(Ixe2x88x92S(STS)xe2x88x921ST)H(HT(Ixe2x88x92S(STS)xe2x88x921ST)H)xe2x88x921HT(Ixe2x88x92S(STS)xe2x88x921ST)y)"sgr"2
where y corresponds to a selected portion of the spread spectrum signal, H corresponds to an interference code matrix for the first signal segment (which defines a first space including the first signal), S corresponds to the interference code matrices for signals of all of the other sources (users) in the selected portion of the spread spectrum signal (which defines a second space including the signals of the other sources), T corresponds to the transpose operation, I is the identity matrix, and "sgr"2 corresponds to the variance of the magnitude of the noise in the selected portion of the spread spectrum signal. Where noise is present, a substantial portion of the noise may be generated by the receiver.
The system can have a number of advantages, especially in spread spectrum systems. The system can significantly increase the signal-to-noise ratio (SNR) of the spread spectrum signal relative to conventional spread spectrum demodulating systems, thereby increasing the detection probability. This is realized by the almost complete removal (i.e., nulling) from the spread spectrum signal of interference attributable to spread spectrum signals generated by other sources. Non-orthogonal (oblique) projections are optimum for nulling structured signals such as spread spectrum signals. In CDMA systems, the system can efficiently demodulate/decode spread spectrum signals generated by far away (weak) sources in the presence of spread spectrum signals generated by near (strong) sources, thereby permitting the base station for a given level of signal quality to service more users and operate more efficiently. An improvement in SNR further translates into an increase in the user capacity of a spectral bandwidthxe2x80x94which is a scarce resource. Unlike conventional systems, the system does not require beam steering to remove the interference.
In applications where the first signal includes a number of multipath signal segments, the system can include a threshold detecting device, in communication with the oblique projecting device, for generating timing information defining a temporal relationship among the plurality of multipath signal segments (e.g., using mathematical peak location techniques that find the points at which the slope of the surface changes from positive to negative and has a large magnitude) and a timing reconciliation device for determining a reference time based on the timing information (i.e., the multipath delays). Multipath signal segments correspond to the various multipaths followed by a signal (e.g., the first signal) after transmission by the signal source.
The system can include a RAKE processor in communication with the oblique projecting device and the timing reconciliation device for aligning the plurality of multipath signal segments in at least one of time and phase and/or scaling the magnitude(s) of the multipath signal segments. RAKE processing rapidly steers the beam of a multi-antenna system as well as mitigates multipath effects. The RAKE processor preferably aligns and scales using the following algorithm:             y      R        ⁡          (      K      )        =            1                        ∑                      i            =            1                    p                ⁢                  A          i                      ⁢                  ∑                  i          =          1                p            ⁢                        A          i                ⁢                  e                                    -              j                        ⁢                          xe2x80x83                        ⁢            φ            ⁢                          xe2x80x83                        ⁢            i                          ⁢                  y          ⁡                      (                          k              +                              t                i                                      )                              
where p is the number of the multipath signal segments (or peaks); i is the number of the multipath signal segment; Ai is the amplitude of ith multipath signal segment; j is the amount of the phase shift; xcfx86i is the phase of the ith multipath signal segment; y(k) is the input sequence; and ti is the delay in the received time for the ith multipath signal segment.
The RAKE processor effectively focuses the beam on the desired signal source. The oblique projecting device and RAKE processor null out the signals of other sources in the spread spectrum signal and thereby eliminate the need for null steering to be performed by the antenna. The system of the present invention is less complex and more efficient than conventional beam steering systems.
The system can include a demodulating device in communication with the RAKE processor to demodulate each of the signal segments. Like the oblique projecting device, the demodulating device uses the equation noted above with respect to the oblique projecting device. Unlike the oblique projecting device which uses portions of the filtered signal to perform oblique projection, the demodulating device uses the output of the RAKE processor which has aligned and summed all of the multipath signal segments. Both the oblique projecting and demodulating devices use estimates of the transmission time (xe2x80x9ctrial timexe2x80x9d) and symbol (xe2x80x9ccandidate symbolxe2x80x9d) and the receive time in determining a correlation function using the above equation. xe2x80x9cReceive timexe2x80x9d is the index into the received data stream (or spread spectrum signal) and represents the time at which the data (or spread spectrum signal) was received by the antenna. xe2x80x9cTransmission timexe2x80x9d is the time at which the source transmitted a selected portion of the data stream (i.e., the selected signal).
In another configuration, the system includes a plurality of antennas (i.e., an antenna array), with each antenna having a respective oblique projecting device, threshold detecting device, and RAKE processor. A common timing reconciliation device is in communication with each of the respective threshold detecting devices and RAKE processors. A common demodulating component is also in communication with each of the RAKE processors. In this configuration, the demodulating component sums all of the first signals received by each of the antennas to yield a corrected first signal reflecting all of the various multipath signal segments related to the first signal.
In either configuration, the system can effectively accommodate the various multipath signal segments related to a source signal. The RAKE processor weights each of the multipath signal segments in direct relation to the magnitude of the peak defined by each multipath signal segment.
The above description of the configurations of the present invention is neither complete nor exhaustive. As will be appreciated, other configurations are possible using one or more of the features set forth above.